Expandable core assembly

ABSTRACT

An expandable core assembly for use in an injection molding operation designed to produce hollow plastic articles of manufacture. The core assembly is positioned between injection mold halves and expanded to define a cavity within an article which is molded between the injection mold halves. After molding, the core assembly is collapsed for convenient removal from within the mold halves and molded article.

[111 3,731,90l May a, 1973 Unite States Patent [1 1 Holdredgejr.

[56] References Cited [54] EXPANDABLE CORE ASSEMBLY T m m N u m m n "Na9 U y we a w Ur i. m3 T ude Am S RAV .0 D m B 0 4 T 505 & I. 999 N 1111m m] 742 P 777 We n 86 r 067 mo n 2 2 PA L8 Vm nd D cwwi wan m u em u m7 s 0 d u [2 3 011 l n "m 7 mmfi Clnxw 9 5 8mm 2 5 cC nvflfr. r. 8 I% a2 ElNN M 1 2 r m N n .m l e m w I F A .l. 11] 6 2 l 7 2 2 r... [.l

ABSTRACT An expandable core assembly for use in an i Related US.Application Data [63] Continuation-impart of Ser. No. 12,812, Feb. 19,

1970, Pat. No. 3,613,605. n ectlon molding operation designed to producehollow plastic articles of manufacture. The core assembly is posipandedto [52] US. Cl....................249/180, 425/417,425/817 51 Int. Cl.7/30 between halves and ex 58 Field ofSearch......................425/l90, 192, 414, define a cavity withinarticle which is l 425/417, DIG. 10; 249/180 between the injection moldhalves. After molding, the core assembly is collapsed for convenientremoval from within the mold halves and molded article.

12 Claims, 12 Drawing Figures PATENTED' 3, T3 1 9'0 1 SHEETIUFS FIG.|

INVENTOR ERNEST c. HOLDREDGE ,JR.

PATENTED MAY 8 I973 SHEET 2 [IF 5 INVENTOR .ERNEST c. HOLDREDGEIJR BY Kymw l f PATENTEU AY 81073 SdEET 3 BF 5 PATENTEDHAY ems SHEET 4 BF 5EXPANDABLE CORE ASSEMBLY CROSS-REFERENCE TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of my copending application Ser.No. 12,812, filed Feb. 19, 1970 and issued as US. Pat. No. 3,613,605.

This invention relates to an expandable core assembly and method for usethereof. More particularly, 10

the present invention concerns a core assembly which may be positionedbetween two injection mold halves to define a cavity within an articleto be molded within the mold defined by the mold halves.

When molding plastic articles in an injection process, hollow articleshaving internal cavities have been prepared in essentially twoprocesses. The first and least desirable process involves manufacture ofarticle halves which are joined together in an operation external of theinjection molding machine. A second and more desirable process involvesthe use of multiple insertable cores which are basically immobile afterinsertion and serve to define a cavity equal to their stationary shape.The multiple cores are individually inserted into slots in the moldhalves prior to plastic injection and are individually removed afterplastic injection and cooling of the molded article. As should beapparent, the second process involves expensive and complicatedequipment and is a time consuming process involving relatively longperiods of down time for the injection molding machine.

It is an object of the present invention, therefore, to provide a methodand apparatus for injection molding of hollow plastic articles whichinvolves the use of an expandable and contractable core assembly todefine multiple cavities or cavities having an area larger than the areaof the collapsed core assembly..

This and other objects, features and advantages of the present inventionwill become apparent from consideration of the following specificationwhen taken in conjunction with the accompanying drawings in which:

FIG. 1 is a somewhat schematic side-elevational view of an openinjection molding apparatus showing two mold halves with a core assemblypositioned therebetween;

FIG. 2 is a front-elevational view of one core half with two coreassemblies in position, and showing a molded pallet in cross-section;

FIG. 3 is a partial perspective view of a core assembly in accordancewith the present invention, the core assembly being in an expandedcondition;

FIG. 4 is an exploded perspective view of a core assembly as shown inFIG. 3;

FIG. 5 is a partial cross-sectional view of a core assembly as shown inFIG. 3;

FIG. 6 is a cross-sectional view taken along the line 6-6 in FIG. 5;

FIG. 7 is a cross-sectional view taken along the line 7-7 in FIG. 5;

FIG. 8 is a cross-sectional view taken along the line 8-8 in FIG. 5;

FIG. 9 is a view similar to FIG. 5 but showing the core assembly in apartially expanded condition;

FIG. 10 is a cross-sectional view taken along the line l0l0 in FIG. 9;

FIG. 11 is a cross-sectional view taken along the line 11 -11 in FIG. 9;and

FIG. 12 is a cross-sectional view taken along the line ll2-I2 in FIG. 9.

Referring now more particularly to the drawings, and to that embodimentof the invention shown by way of illustration, it will be seen in FIG. 3that the core assembly includes a front plate 10 and a rear plate 11disposed parallel to the front plate. A right-hand end cap 12 and aleft-hand end cap 13 are disposed parallel to each other along theright-hand and left-hand edges respectively of front and rear plates 10and 11. Spaced along end caps 12 and 13 are projecting bosses, l4 and 15on end cap 12 and I6 and 18 on end cap 13. Extending from the center ofthe core assembly is an actuating power wedge plate 19.

Referring now to FIG. 4 of the drawing for a better understanding of theconstruction of the core assembly, it will be understood that the frontplate 10 is substantially identical to the rear plate Ill, and theleft-hand end cap 13 is substantially identical to the right-hand endcap 12. In FIG. 4;, the left-hand end cap 13 and the front plate 10 areomitted for purposes of clarity.

The rear plate 11 has a plurality of recesses 20 along its insidesurface, each of the recesses 20 being adapted to receive a wedge insert21. As here shown, there are four rows of wedge inserts. Along the fulllength of the rear plate 11 are rails 22 and 24, each rail having threerear plate cam pins 25 projecting perpendicularly therefrom. It shouldbe understood that when assembled the front plate 10 is disposedparallel to the rear plate 11 with its wedge inserts 21 facing the wedgeinserts 21 of the rear plate ll, and having similar pins 25.

Positioned between the front plate It) and the rear plate 11 isactuating plate 19 which includes a plurality of wedge surfaces 28. Ashere shown, there are four wedge surfaces 28 to correspond to the fourrows of wedge inserts 21. This arrangement will be discussed more fullyhereinafter.

Along the right-hand and left-hand edges of actuating power wedge plate19 are cam support shoulders 29 and 30 which are joined to the main bodyof actuating plate 19 by connecting webs 31 and 32 respectively. The camsupport shoulders carry cam blocks 34 and 35 on their rear and frontsurfaces respectively, the cam blocks 34 and 35 being aligned with eachother and secured by means of bolts or the like to the cam supportshoulder 31. The left cam support shoulder 31 has three sets of camblocks 34 and 35 spaced along its length; and, the right cam supportshoulder 30 has complementary blocks 36 and 38 spaced along its lengtharranged similarly to the cam blocks 34 and 35.

Each of the cam blocks 34, 35, 36 and 38 has a cam slot 39 along itslength with the slots 35 facing towards the center of actuating plate19. As will be more fully understood later, the pins 25 on the front andrear plates 11 are received within the slots 39 to cause contraction ofthe front and rear plates 10 and II when appropriate.

Projecting outwardly from cam support shoulders 31 and 30 arespaced-apart cam pin necks 4s, 41, 42 and 44 respectively. Each of thesecam pin necks 40, M, 42 and d4 has a semicircular recess in its distalend to receive cam pins 45, 46, Q8 and 49. The cam pins 45, 4s, 48 and49 are held in place by appropriate cam pin caps indicated at 50.

Referring now to the end cap 12 in FIG. 4 of the drawings, it will beseen that the cap is basically a channel-shaped member having a centralweb 51 with front and rear flanges 52 projecting inwardly therefrom.Slots 54 and 55 extend through the central web 51 and through the bosses14 and respectively. Slots 54 and 55 are adapted to receive identicalend cap cam members 56 and 58 respectively. The cam member 56 has theshape of the Greek letter Pi when viewed in crosssection and has opposedcam passages 59 defined in each of its legs. Referring to FIG. 8 of thedrawings, it will be seen that the boss 14 is spot-faced to receive themounting plates 60 of the cap cam member 56 so that the member 56 can beinserted into the boss with the legs of the cam member 56 extending intothe slot 54. The cam pin 48 rides within the opposed cam passages 59. Itshould again be mentioned that the member 56 is typical of theconstruction at all four of the bosses 14, l5, l6, and 18, each havingsimilar cam passages with the cam pins 45, 46, 48 and 49 riding alongthe cam surfaces of the passages.

Referring now to FIG. 4, it may be seen that end cap 12 includes threepairs of spacer blocks 61, 62 and 64 attached thereto. There is acomplementary set of spacer blocks attached to the end cap 13 for eachof the pairs of spacer blocks 61, 62 and 64.

At this point, it should be understood that when the core assembly isproperly assembled the actuating power wedge plate 19 is sandwichedbetween the front and rear plates 10 and 11 with the end caps 12 and 13substantially adjoining the right-hand and left-hand edges of the frontand rear plates. The pins 25 are received within the slots 39 of the camblocks 34, 35, 36 and 38 so that, if we assume that the front and rearplates 11 are vertically fixed, as the actuating plate 19 is movedinwardly and outwardly the slots 39 in the cam blocks traverse the pins25 and cause a motion of the pins 25 and their side-plates in accordancewith the slope of the cam slots 39 in the cam blocks 3 3-38.Simultaneously, it should be understood that as the actuating plate ismoved inwardly the wedge surfaces 28 engage the wedge inserts 21 to urgethe wedge inserts 21 and their side-plates away from the actuatingplate. The combination of the motion caused by the wedge surfaces actionagainst the wedge inserts 21 and the motion caused by the pinstraversing the cam slots 39 causes the front and rear plates 10 and 11to move outwardly and inwardly in response to the movement of theactuating plate 19. It should be understood that the motion caused byeach of these actions is complementary.

In addition, as the actuating plate 19 moves inwardly and outwardly, thecam pins 45, 46, 48 and 49 ride along the surfaces of cam passages 59and, since these cam surfaces are in effect fixed to end caps 12 and 13,the end caps are moved laterally as the cam pins move along thesurfaces.

An interesting aspect of the present invention concerns the constructionmaterials selected for the present embodiment of this invention. Alightweight aluminum alloy is chosen for the core-assembly front andrear plates and the core-assembly end caps. The alloy is selectedprimarily because of its high thermal conductivity and freemachineability. In addition, the alloy is sufficiently flexible so thatapplication of plastic material under pressure to the core assembly andparticularly the core-assembly end caps causes fins 52 on the end capsto deflect inwardly and seal against surfaces of the front and rearplates. Such a seal prevents the undesirable entry of plastic materialinto the internal mechanism of the core assembly. Ferrous alloys couldbe used for these parts and it would not be necessary to provideinternal supports for applied stresses.

Hardened steel of two primary hardness levels is used for the heavilyworked parts such as the powerwedge plate, the core side-plate cams, theend-cap cams, the end-cap cam pins, the front and rear plate cam pinsand the wedge inserts. Hardnesses have been chosen for these parts toallow adjustment and/or replacement of the more easily manufactureditem.

OPERATION The operation of the present core assembly is preferablyperformed in conjunction with an appropriately designed injectionmolding apparatus. Such an apparatus conventionally includes astationary mold half, a moveable mold half and a plastic material supplyapparatus such as an extruder. In the present operation embodiment, afoamed plastic material and its associated preparation equipment isemployed. Such a construction material has been found to be mostdesirable for use with the present core assembly in the construction offoamed thermoplastic pallets having a hollow interior cavity.

A discussion of the internal movements and operations of the coreassembly is presented initially to facilitate an understanding of theoverall operation of the assembly and associated injection moldingequipment. For convenience of discussion, motions of end caps 12 and 13and motions of front and rear plates 10 and 11 will be taken one at atime. It should be understood, however, that the motions occursimultaneously during the operation of the assembly. With reference toFIG. 5 of the drawings, it will be seen that the end cap 12 is in apartially retracted position. If it is considered that the apparatus asshown in FIG. 5 is moving in the direction of retracting end cap 12, theactuating plate 19 will be moving upwardly in the drawing. The end capwill be stationary in the vertical direction and the actuating plate 19together with shoulder 30 and its attached cam pin neck 42 and cam pin48 will be moving upwardly. The cam pin 48 will ride along the surfaceof cam passage 59 and the end cap 12 will be drawn inwardly towards theactuating plate 19, or to the left as viewed in FIG. 5.

Assuming now that the end cap 12 is to be extended to place the coreassembly in its expanded position, the actuating plate 19 will be moveddownwardly as viewed in FIG. 5 to cause cam pin 48 to move down, ridingalong the surface of cam passage 59. As the cam pin 48 moves down alongthe cam surface, the end cap 12 will be moved outwardly; further, itwill be seen that the entire outward movement will take place with onlya short movement of the cam pin 48. After the end cap 12 has moved outto its full extent, the cam pin 48 will ride along the straight portionof the cam surface 59 which will cause no further outward movement ofend cap 12, but the additional motion is necessary to arrange theinterior supports properly and provide continuous support and outwardmotion of the side plates 18 and 11 through wedge inserts 21 which alsoride on actuating plate 19.

Since, as described so far, the only retaining support for end caps 12and 13 is the connection by means of the cam pin 48, the end caps 12 and13 might not withstand full molding pressure without substantialdeflection or failure as encountered in a large injection moldingapparatus such as shown in the drawing, as the cam pin 48 moves alongthe straight portion of the cam surface 59, the cam blocks 36 and 38will move into an abutting relationship with thespacer blocks 61. Sincespacer blocks 61 are rigidly attached to end cap 12, and cam blocks 36and 38 are rigidly attached to actuating plate 19, forces exertedagainst the outside of end cap 12 will be transmitted to the solidmaterial of the actuating plate and supported thereby.

In this connection, it will be noted that the spacer blocks 61, 62, and64 are located on each side of the bosses l4 and 15. As will be seenlater, the areas of the bosses 14 and 15 do not receive the moldingpressure, so no additional support in these areas is necessary.

Motion of the front and rear plates 10 and 11 will now be considered andattention is directed primarily to FIGS. 6 and 10 of the drawings.

It will be seen that the cooperation of the pins 25 with the cam blocks34, 35, 36, and 38 serves a multiple function. Since the front and rearplates 10 and 11 must move outwardly with respect to the actuating plate19, some means must be provided to hold the assembly together, yet toallow for the necessary motion. Since each of the plates 10 and 11 hasthe extending pins 25 and there is a cam block such as the cam blocks 36and 38 on each side of the wedge plate 19, one must simply hold the twowedge blocks 36 and 38 together, and the pins 25 will hold the plates 10and 11 to the cam blocks 36 and 38. To allow for the necessary motion,the cam blocks 36 and 38 have slots 39 which are slanted to act as a camtrack. As the actuating plate 19 is moved upwardly, the cam blocks 36and 38 move upwardly and the slant of cam tracks 39 causes the pins 25to move towards each other and cause retraction of the front and rearplates 10 and 11. As the actuating plate 19 is moved downwardly, thefront and rear plates 10 and 11 are driven apart by plate 19 and thepins 25 idle along cam tracks 39 in blocks 36 and 38. The retractedposition is shown in FIG. 6 of the drawings and the expanded position isshown in FIG. 18 of the drawings.

Again, considering the extreme pressures involved in an injectionmolding process, the arrangement described above with the pins 25 ridingin the cam slots 39 would be insufficient to withstand the forces. Also,the core assembly of the present invention is contemplated for use inrelatively large moldings so that the core assembly itself would bequite heavy. To provide the motivating force for expanding the front andrear plates 10 and 11, the wedge surfaces 28 on the actuating member 19are used in conjunction with the wedge inserts 21. Referring to FIG. 7of the drawings it will be seen that the front and rear walls 18 and 11are in their retracted positions and the narrower portion of theactuating member 19 is abutting the wedge inserts 21. As the actuatingmember 19 is=moved downwardly, the wedge inserts 21 on each side of theactuating member 19 are forced apart to expand the front and rear walls18 and 11, moving the two plates away from each other. FIG. 11, then,shows the front and rear plates 10 and 11 in their expanded positionwith the thickest portion of the actuating member 19 between the wedgeinserts 21. In this position, forces exerted on the front and rearplates 10 and 11 are opposed through solid metal rather than dependingupon such relationship as pins 25 riding in slots 39.

A further consideration for a core assembly for molding, and especiallythe molding of plastics, concerns cooling of the molded article to asolid condition. Cooling of the core assembly of the present inventionis provided by coolant passageways and 71 extending longitudinallythroughout the end cap 12, two two passageways 78 and 71 being connectedat the bottom of the core assembly by a U-tube 72. Similar passageways7d and 75 are defined in end cap 13 and connected by a U-tube 76 at thebottom of the core assembly. The front and rear plates 10 and 11 havecoolant passages extending longitudinally throughout their length, thesepassageways being designated 78, 78, 79, 79', 88 and 80. At the bottomof the core assembly are U-tubes 78" connecting coolant passageways 78and 78', U-tube 79" connecting passageway 79 and 79", and U-tube 88"connecting passageways 80 and 80. It will thus be seen that coolant maybe directed through the core assembly to cool a molded article.

Attention is now directed to FIG. 1 of the drawings showing the coreassembly in position in an injection molding machine. The core assemblyis held in place with respect to the molding machine by the use of pipenipples threadedly received into the top openings of the coolantpassageways such as passageways 70, 71, et seq. These pipe nipples arethen held by appropriate means to bearing blocks such as the bearingblock 83 shown in FIG. I, and appropriate tubing is connected to thepipe nipples to carry the coolant to and from the core assembly.

The injection molding machine shown in FIG. 1 is a conventional machinehaving a stationary mold half 81 and a moveable mold half 82 opposed toeach other and aligned for proper mating. The core assembly A issuspended between the two halves 81 and 82 so that when the two moldhalves are brought together to form a closed mold, the core assembly Ais within that closed mold.

Though numerous physical arrangements may be employed themselves tosuspend the core assembly A for proper operation, one suitable form ofapparatus is here illustrated. A block 84 is mounted on the stationarymold half, and rods such as rod 85 are fixed to block 84. Bearingblocks, such as block 86, are then fixed to the moveable mold half withrods 85 slideable within bearing blocks 86. Block 83 is also a bearingblock that is slideable along rods 85, and it is from block 83 that thecore assembly A is suspended. Also carried by bearing block 83 is afluid actuated cylinder 88 having its actuating rod (not shown) attachedto the actuating plate 19 for operation thereof. The fluid actuatedcylinder 88 is braced through braces 89 to a further bearing block 98which is also slideable on rods 85.

An operation cycle will now be disclosed. Since the item to be molded inthis illustration is a pallet in which fork holes are provided on allfour sides, two of the core assemblies A are required. The relationshipof these core assemblies to the mold halves 81 and 82 is shown in FIG. 2of the drawings. From this figure it will be seen that the bosses 14,15, 16 and 18 on each of the core assemblies extend beyond the actualmolding cavity and into recesses in the mold to define the side forkholes in the pallet structure. For ease of explanation, only one coreassembly will be mentioned and discussed in the operating cycle. Itshould be apparent that the additional assembly will operate in likefashion.

At the initiation of the cycle, the press mold is at its maximum openposition and a collapsed core assembly is suspended between the two moldhalves. The operation of the core assembly is automatically controlledfrom an auxiliary panel which is interconnected with the press andextruder controls. Switches are activated on the control panels (notshown) and the moveable mold half initiates closing movement. Themoveable half 82 engages the core assemblies A which seat therein, thuscausing the bearing blocks 83 and 90 to slide along rods 85 so that theentire apparatus is moved towards the stationary half 81. Though thecore assembly A is very long, providing a long lever arm, the inertia ofinitial movement is somewhat overcome by the presence of the fluidactuated cylinder 88 mounted in alignment over the assembly. Since thefluid actuated cylinder 88 is rigidly tied to the bearing block 90, asubstantial resistance is provided to prevent swinging of the coreassembly A.

When the core assembly seats in the moving mold half, a limit switch isactuated which energized a cylinder 88 to drive the actuating wedgeplate into the core assembly and cause the front and rear plates tospread due to the taper of the wedge plate work surfaces 28 riding onthe wedge inserts 21. At the beginning of the actuating wedge platestroke, and through the first 25 percent of its motion, the cam pins 45,46, 48 and 49, riding in the end-cap cams 56 and 58, begin their motionalong the cam surfaces 59 causing the end-caps l2 and 13 to moveoutward. The cam pins then idle along in the end-cap cam surfaces 59through the remaining 75 percent of the stroke. Through correct choicesof cam surfaces, the actuating wedge plate, at the end of its stroke,seals the front and rear plates to the end-caps. At this point, the coreassembly is both indexed in the moving mold half and fully expanded withthe moving half continuing toward the closed position. The front andrear plate cam pins 25 have idled along to the expanded position,although supplying no useful opening work at this point.

As the moving mold half and core assembly approach the fixed mold halfto achieve a fully closed position, a safety switch is actuated. Thefunction of the switch is to determine l that the core assembly iscorrectly indexed in the moving mold half; and (2) that the coreassemblyis fully expanded and locked. If either of these sequences has not takenplace, the cycle is aborted and the entire mechanism including the pressreturns to a neutral position.

As the mold halves and core assembly meet, the fixed mold half of thepress is considered closed and resistance to molding pressure isdeveloped by the clamp portion of the press. A limit switch is thenactuated to indicate that resisting force has been developed and thatthe press, in fact, is closed which enables plastic injection to takeplace.

In order for the core assembly to function properly and generate theinside surface of the molded part, the assembly must be capable ofresisting forces generated by the molten plastic as it is forced intothe cavity of the mold. The failure of the core assembly to resist thisforce would enable molten plastic to enter between the working surfacesof the core assembly where it would then solidify rendering the coreassembly unusable. Therefore, certain devices are incorporated in thecore assembly to resist this force. In addition to the cam pins ridingin the end-cap cams, spacer blocks 61, 62 and 64 are provided to restagainst the back of cam blocks 34, 35, 36 and 38 during the expandedportion of the cycle to resist the tendency for the end-caps to deflectunder molding pressure. These spacer blocks are located such that theyclear the front and rear plate cam blocks during the first percent ofthe actuating wedge plate stroke and then extend adjacent the cam blocksduring the last 25 percent of the stroke. The spacer blocks coupled withcam pins 45, 46, 48 and 49 and end-cap cam surfaces 59 provide theresisting force to prevent end-cap collapse. The front and rear sideplates contain wedge insert recesses 20 which contain wedge inserts 21that bear against the working surfaces of the actuating wedge plate.These wedge inserts bear uniformly over the surface of the wedge plateand coupled with the hydraulic locking of the wedge plate in the maximumdown position, provide the resisting force for the front and rearplates.

At this point, plastic is injected into the space defined between theinside of the fixed and moving mold halves and the outside of the coreassembly. The clamp portion of the press resists the tendency for thefixed and moving mold halves to spread during the application of moldingpressure and the hydraulic cylinders above provide the resisting forceto prevent core assembly collapse.

Coolant is then circulated through the core assembly and the moving andfixed mold halves, while the entire assembly is held in the closedposition, and plastic solidification occurs.

When the part is completely solidified, a timer initiates opening of thepress and movement of the core assembly, the moving mold half and themolded part moving away from the fixed mold half. The molded part is,therefore, stripped or separated from the fixed mold half.

The molded part, core assembly and moving mold half continue toward theopen position until such time as the core assembly and the molded partstrike knockout rods 96 which cause the core assembly to come to restand the moving mold half to-continue opening, thus stripping orseparating the core assembly and the molded part from the moving moldhalf.

As the moving mold half separates from the core assembly, a limit switchis actuated causing the hydraulic cylinder 88 to withdraw the actuatingwedge plate from the core assembly. Two operational sequences then occursubstantially simultaneously which enable the core assembly to collapse:(1) the front and rear plate cam pins 25 riding in cam blocks 34, 35, 36and 38 pull the side plates inward, separating the side plates from theinside surface of the molded part. This action occurs uniformly through100 percent of the wedge plate stroke; (2) through the first 75 percentof the wedge plate motion the abutting relationship of the cam blocksand spacer blocks is removed and in the last 25 percent of the wedgeplate motion the cam pins 45, 46, 48 and 49 riding in the end-cap earns56 and 58 pull the end-caps inward, separating the end-caps from themolded part. As the core assembly approaches the collapse position, themolded part P is freed from the core assembly and falls off the core. Inthis operational embodiment, a pit is provided beneath the core assemblyfor reception of the pallet.

Following expansion of the core assembly and mating of the core halves,plastic material is prepared and injected into the mold cavity accordingto the following steps: (a) melting and mixing a mixture of a blowingagent and a thermoplastic material in an extruder at a temperature andpressure above the foaming temperature and pressure of the mixture; (b)continuously extruding the mixture into an expanding accumulation zonein continuous communication with the extruder which zone expands at arate substantially equal to the rate of extrusion while maintaining themixture therein in the molten state and at a pressure above the foamingpressure thereof; maintaining the pallet mold at a pressure no greaterthan the pressure at which the mixture foams and expands, and preferablyat about atmospheric pressure; (d) establishing communication betweenthe mold and the accumulation zone; (e) rapidly passing the mixture outof the accumulation zone and into the mold cavity whereby the pressuredifferential between the accumulation zone and the mold cavity causesthe mixture to rapidly expand in the mold; and (f) terminatingcommunication between the mold cavity and the accumulation zone at apoint when the accumulation zone is empty.

An improved overall apparatus suitable for carrying out the process formolding foamed thermoplastic pallets comprises (a) extruder meansadapted to melt, mix and continuously extrude a mixture of a blowingagent and a thermoplastic material at a temperature and pressure abovethe foaming temperature and pressure of the mixture; (b) an expandableaccumulation chamber in continuous communication with the extruderadapted to receive the mixture from the extruder and expand at a ratesubstantially equal to the rate of extrusion and having means formaintaining the mixture therein in the molten state and at a pressureabove the foaming pressure thereof; (c) a pallet mold with expandablecore assembly adapted to be maintained at a pressure no greater than thepressure at which the mixture foams and expands and preferably at aboutatmospheric pressure; (d) means for establishing and terminatingcommunication between the mold and the accumulation chamber at a pointwhere the pressure is above the foaming pressure of the mixture; (e)means for rapidly passing the mixture out of the accumulation chamberand into the mold when communication is established therebetween.

Unexpectedly, it has been found that the process of this invention notonly produces a dense skin region and a cellular core region in thestructural elements of the pallet but can also produce an isotropicproduct free of internal stresses and distortions even in molds ofcomplex shape and thick sections at room temperature or less.

In one embodiment of this process, an accumulator device is providedwhich includes cylinder means having at one end thereof, communicationmeans for the ingress of material into the cylinder from an extruder andfor the egress of the material from the cylinder into a mold remotetherefrom, and a piston reciprocally movable therein between a firstposition proximate said one end of the cylinder and a second positiondistant from said one end. Pressure means are also provided for applyingforce to the piston member in the direction of said one end. Associatedwith the cylinder means are valve means for establishing and terminatingcommunication between a mold and the cylinder at a point remote from themold and for forcing material between it and the mold into the mold.Advantageously, means are provided for automatically actuating the valvemeans. Further, the means can be responsive to the movement of thepiston. Thus, when the piston is in the first position, the valve meanscan be actuated to terminate communication between the mold and thecylinder means, and when the piston is in its second position, the valvemeans are actuated to establish communication between the mold andcylinder means.

Thermoplastic polymeric materials in general can be molded according tothe present process. Particularly preferred materials include high andlow density polyethylene and polypropylene. Other suitable resinsinclude ethylene-vinyl-acetate copolymers, rubber modifiedpolypropylene, ethylenepropylene copolymers, crystalline polystyrene,high-impact polystyrene, acrylonitrile-butadiene-styrene polymers, vinylresins, nylons, polyarylene polyethers, polyhydroxyether, polycarbonate,polysulfone, polyesters such as polyethylene terephthalate, celluloseesters and cellulose ethers such as cellulose acetate and celluloseacetate butyrate; and polyvinyl chloride.

The specific gravity of molded pallets may be varied from 0.4 to that ofthe solid resin but the best balance for toughness and stiffness isgenerally found in pallets that have a specific gravity range of0.55 to0.75.

The mixture employed in the process can also contain molding additivessuch as heat and light stabilizers, pigments, mold release agents,natural and synthetic fibers and slip agents. A conventional nucleatingagent, such as calcium silicate, may be added to the mixture in order toenhance the uniformity of the cells produced by the foaming action. Theingredients of the mixture can be premixed, if desired, and charged toan extruder wherein the thermoplastic material is fluxed and mixed.

The blowing agent can be selected from chemical compounds such as azo,N-nitroso, carbonate and sulfonyl hydrazide compounds which are heatdecomposable to yield a gas such as carbon dioxide or nitrogen. Theblowing agent can also consist of normally gaseous agents such as methylchloride, propylene, butylene and gaseous fluorocarbons, as well asgases such as nitrogen, carbon dioxide or air. Volatile liquids such aspentane, water and liquid flurocarbons can also be employed in thefoaming of the polymer.

While the various types of blowing agents which can be employed toproduce a foamed product can be mixed with the plastic material atvarious stages in the process, it is advantageous to add liquid orgaseous agents directly to the polymeric material in the extruder, whilethe thermoplastic material is in a molten state in order to obtain auniform dispersal of the agent within the molten plastic withoutemploying additional mixing apparatus. Similarly, a decomposablechemical blowing agent is advantageously premixed with the polymer priorto the charging of the polymer into the extruder.

Careful consideration should be given to the means of obtaining ahomogenous distribution of the decomposable chemical foaming agent inthe plastic compound. This can be conveniently achieved by adding thefoaming agent in the form of solid particles or in the form of adispersion in a vehicle compatible with the basic resin. The mixing isthen carried out in conventional equipment such as a churn, colloidmill, threeroll mill, or Banbury, the choice of which depends upon theviscosity of the plastic compound. Alternatively, to obtain uniformdispersion, pellets of plastic resin can be coated with the blowingagent by tumbling. Regardless of the method used, the foaming agent mustbe incorporated below its decomposition temperature to prevent thepossibility of gas losses taking place in the plastic mass prior toexpansion.

In the case of chemical blowing agents premixed with the polymer thetime-temperature relations within the extruder must be adequate todecompose the blowing agent to release the gas within the extruder. Theextruder pressure must then be maintained uniformly high at least untilthe mixture is transferred into the high pressure accumulator in orderto prevent premature expansion of the cells.

The decomposition of the blowing agent can be delayed through the propercontrol of the extruder operating temperature and/or through the properselection of the blowing agent. Thus, for example, diazoaminobenzene canbe employed for decomposition at about 95 to 105 C. whileazodicarbonamide can be employed for decomposition at the highertemperature range of 160 to 200C.

The mixture being charged into the expanding zone must be maintained ata pressure above the foaming pressure thereof. While a pressure of aslow as about 500 p.s.i. will normally prevent premature foaming of themixture, pressures of at least 1,500 p.s.i. have been found to give bestresults. Pressures in excess of 10,000 psi. are usually not required forproper operation of the accumulator device and normally should beavoided because operating expenses increase with increasing pressures.

in addition, the cylinder means is provided with heating means tomaintain the mixture in the molten state and thus prevent solidificationof thermoplastic material therein. It should be noted, however, that noheating of the mixture in the cylinder is required. Heat is applied tothe mixture in the extruder and is only maintained in the cylinder.

The temperature of the mold is not critical. Cold molds, e.g. at roomtemperatures, about C can be used to produce foamed pallets having adull mottled surface similar to the texture and appearance of wood.Lower mold temperatures are preferred since the time required forcooling the article in the mold is shorter. Higher mold temperaturespermit more flow of the molten mixture in the mold prior to cooling thanwith a cooler mold and produce a smooth, glossy or glazed mottledsurface.

Molds which cause sharp pressure drops to take place therein, due to acomplex configuration, or sharp corners or the like, require the use ofa greater amount and pressure of blowing agent in order to achieve aparticular degree of foaming.

From the foregoing description, it is evident that the process andpreferred apparatus of this invention can be employed to rapidly,efficiently, and automatically mold foamed thermoplastic pallets on abatch, semicontinuous or continuous basis.

It is to be understood that while the mixture in the accumulation zoneis maintained at a substantially constant pressure, this pressure beinggreater than the pressure at which the blowing agent causes the foamingand expansion of the foamable material, pressures which would absolutelyprevent foaming are unnecessary in the instant invention for minorbubbling is not significant and is not detrimental to the resultantproduct.

In the present process there are two forces driving the plastic materialout of the accumulator. One is the direct pressure exerted by theaccumulator and the other is the force of the gas expanding as ittravels from the high pressure accumulator to the low pressure mold.Accumulator pressures are generally between 1,000 and 3,500 psi. whilethe mold pressure is low since it is vented along the parting line. Inone embodiment only enough solid plastic to fill about one half of themold is delivered by the accumulator, but since the solid plasticcontains gas it expands and fills the mold with foam. As the foam flowsthrough the mold the surface cells collapse and solid skins are formed.These skins are beneficial since the maximum tensile and compressivestresses occur on the surface when a member is subject to bending.

When the mold is not packed with solid plastic material, the pressureswithin the mold are low, in the range of 50 to 300 p.s.i., and aluminum,beryllium, or other metallic molds can be used. Aluminum molds removeheat from the foamed plastic about twice as fast as steel molds. Rapidheat removal is very important when cooling an insulating type ofmaterial.

It is within the spirit and scope of this invention to usescrew-injection type apparatus, such as that described in US. Pat. No.3,124,841, in conjunction with means as described herein forestablishing and terminating communication between the mold, theaccumulation zone or chamber, and the extruder (screw). The accumulationzone or chamber in this instance is between the extrusion screw and theaforesaid means for establishing and terminating communication. It isalso within the spirit and scope of the present invention to employ themethods and apparatus as disclosed in U.S. Pat. Nos. 3,268,636 and3,436,446.

The phrases foaming pressure" and foaming temperature" as used hereinare defined as follows:

The phrase foaming pressure" refers to the pressure at which aninternally contained gas can produce substantial foaming and expansionof the plastic material within which it is contained.

The phrase foaming temperature" refers to the temperature above which asolid or liquid blowing agent will become a gas at pressures less thanthe foaming pressure.

While this invention has been described in detail with particularreference to preferred embodiments thereof, it should be understood thatvariations and modifications can be effected within the spirit and scopeof the invention as described hereinbefore and as defined in theappended claims.

I claim:

1. An expandable core assembly for use in defining a cavity within anarticle molded within an injection molding machine comprising a moveablewedge plate having wedge surfaces on opposite sides thereof and sideedges; a front and a rear plate disposed on opposite sides of the wedgeplate adjacent the wedge surfaces of the wedge plate, said front andrear plates being moveably joined to the wedge plate; a right and a leftend cap moveably joined to the wedge plate along portions of the sideedges thereof, said right and left end caps including cam tracks andsaid wedge plate including cam pins which are disposed within andmoveable along each of said cam tracks, and means on said wedge plateand said front and rear plates and selectively operative to relieveforce exerted against said core assembly from transmission to said wedgeplate through said cam tracks and cam pins.

2. An expandable core assembly as defined in claim 3 wherein said wedgeplate includes cam blocks along the side edges thereof, said cam blocksdefining cam slots; and said front and rear plates include cam pinswhich are disposed within and moveable along each of said cam slots.

3. An expandable core assembly as defined in claim 2 wherein said endcaps include spacer blocks rigidly fixed thereto, said spacer blocksbeing located so as to abutt said cam blocks when said wedge plate ismoved to one position.

4. An expandable core assembly as defined in claim 1 wherein said wedgeplate includes a first plurality of sloping wedge surfaces on one sidethereof and a second plurality of sloping wedge surfaces on the oppositeside thereof; and said front plate and rear plate have a plurality ofwedge inserts which are located so as to be acted upon by the slopingwedge surfaces of the wedge plate when said wedge plate is moved in afirst direction.

5. An expandable core assembly for use in defining a cavity within amolded article, comprising:

at least one pair of spaced apart mold members each having a firstsurface which defines a portion of the cavity;

an actuating member received within the space between said spaced apartmold members and movably joined to said mold members with means allowingsaid actuating member a predetermined extent of movement with respect tosaid mold members;

first cam means connected with each of said mold members;

second cam means disposed on said actuating member in complementaryoperative relation with said first cam means, said first and second cammeans being operative to extend said mold members to a position ofmaximum spacing responsive to movement of said actuating member to afirst position in said predetermined extent of move ment and beingoperative to retract said mold lid members to a position of lesserspacing responsive to movement of said actuating member to a secondposition in said predetermined extent of movement;

first abutment means disposed on each of said mold members in facingrelation with said actuating member;

second abutment means disposed on said actuating member in facingrelation with said spaced apart mold members, said second abutment meanspositioned on said actuating member to be offset from said firstabutment means when said actuating member is in said second position andto be in abutting relation with said first abutment means to relieveforce exerted against said mold members from transmission to saidactuating member through said cam means when said actuating member is insaid first position.

6. An expandable core assembly as in claim 5, wherein said firstabutment means comprises at least one wedge surface rigidly affixed toeach of said pair of mold members; and said second abutment meanscomprises corresponding wedge surfaces rigidly affixed to said actuatingmember in position to move into wedging abutment with said at least onewedge surface of each of the mold members when said actuating member isin said first position.

7. An expandable core assembly as in claim 6, wherein said firstsurfaces of said mold members are proximately parallel with respect toeach other.

8. An expandable core assembly for use in an injection molding apparatusfor defining a cavity within a molded article, comprising:

a first pair of spaced apart mold members each having a surface whichdefines a portion of the cavity;

a second pair of spaced apart mold members each having a surface whichdefines another portion of the cavity;

an actuating member received within the space between said first andsecond pairs of mold members and moveably joined to each of said moldmembers with means allowing said actuating member a predetermined extentof movement with respect to said mold members, said first and secondpairs of mold members being retained by said actuating member inpredetermined spaced relation so that the space between said pairsdefines a region within which said actuating member is received;

first cam means connected with each of said first pair of mold members;

second cam means disposed on said actuating member in complementaryoperative relation with said first cam means;

third cam means connected with each of said second pair of mold members;

fourth cam means disposed on said actuating member in complementaryoperative relation with said third cam means; said first and second cammeans being operative to extend said first pair mold members to aposition of maximum spacing responsive to movement of said actuatingmember to a first position in said predetermined extent of movement andsaid third and fourth cam means being operative to extend said secondpair of mold members to a position of maximum spacing responsive tomovement of said actuating member to said first position;

said first and second cam means being operative to retract said firstpair of mold members to a position of lesser spacing responsive tomovement of said actuating member to a second position in saidpredetermined extent of movement, and said third and fourth cam meansbeing operative to retract said second pair of mold members to aposition of lesser spacing responsive to movement of said actuatingmember to said second position;

first abutment means disposed on each of said first pair of mold membersin facing relation with said actuating member;

second abutment means disposed on said actuating member in facingrelation with said spaced apart first pair of mold members;

third abutment means disposed on each of said second pair of moldmembers in facing relation with said actuating member;

fourth abutment means disposed on said actuating member in facingrelation with said spaced apart second pair of mold members;

said second abutment means positioned on said actuating member in offsetrelation with said first abutment means when said actuating member is insaid second position and in abutting relation with said first abutmentmeans when said actuating member is in said first position; and

said fourth abutment means positioned on said actuating member in offsetrelation with said third abutment means when said actuating member is insaid second position and in abutting relation with said third abutmentmeans when said actuating member is in said first position; each of saidabutment means in said abutting relation being cooperative to relieveforce exerted against said mold members from transmission to saidactuating member through said cam members.

9. An expandable core assembly as in claim 8,

wherein said surfaces of said first pair of mold members are parallelwith respect to each other, and said surfaces of said second pair ofmold members are parallel with respect to each other, and are positionedat a substantial angle relative to said surfaces of said first pair ofmold members.

10. An expandable core assembly as in claim 8, wherein:

said first and second abutment means comprise at least one set ofcomplementary wedge surfaces rigidly affixed to each member of saidfirst pair of mold members and to said actuating member, respectively;said third and fourth abutment means comprise at least one s t ofcomplementary wedge surfaces rigidly affixed to each member of saidsecond pair of mold members and to said actuating member, respectively;said complementary wedge surfaces on said actuating member beingpositioned to move into wedging abutment with said respectivecomplementary wedge surfaces on said mold members when said actuatingmember is in said first posilll xh expandable core assembly as in claim8, wherein:

said surfaces of said first pair of mold members have edges positionedin adjacent relation with edges of said surfaces of said second pair ofmold members to define a core volume which is substantially enclosedaround said pairs of surfaces;

the surfaces of one of said pairs of mold members including fin edgemeans which closely overlap the adjacent edges of the surfaces of theother pair of mold members when said pairs of mold members are in theextended position;

said fin edge means being resiliently yieldable in response to injectionmolding pressure to move into sealing engagement with said adjacentedges of said other pair of mold members.

12. An expandable core assembly as in claim 8, further comprising fluidflow passageway means disposed within said mold members to guide a flowof coolant fluid in heat exchange relation with said mold members.

1. An expandable core assembly for use in defining a cavity within anarticle molded within an injection molding machine comprising a moveablewedge plate having wedge surfaces on opposite sides thereof and sideedges; a front and a rear plate disposed on opposite sides of the wedgeplate adjacent the wedge surfaces of the wedge plate, said front andrear plates being moveably joined to the wedge plate; a right and a leftend cap moveably joined to the wedge plate along portions of the sideedges thereof, said right and left end caps including cam tracks andsaid wedge plate including cam pins which are disposed within andmoveable along each of said cam tracks, and means on said wedge plateand said front and rear plates and selectively operative to relieveforce exerted against said core assembly from transmission to said wedgeplate through said cam tracks and cam pins.
 2. An expandable coreassembly as defined in claim 1 wherein said wedge plate includes camblocks along the side edges thereof, said cam blocks defining cam slots;and said front and rear plates include cam pins which are disposedwithin and moveable along each of said cam slots.
 3. An expandable coreassembly as defined in claim 2 wherein said end caps include spacerblocks rigidly fixed thereto, said spacer blocks being located so as toabutt said cam blocks when said wedge plate is moved to one position. 4.An expandable core assembly as defined in claim 1 wherein said wedgeplate includes a first plurality of sloping wedge surfaces on one sidethereof and a second plurality of sloping wedge surfaces on the oppositeside thereof; and said front plate and rear plate have a plurality ofwedge inserts which are located so as to be acted upon by the slopingwedge surfaces of the wedge plate when said wedge plate is moved in afirst direction.
 5. An expandable core assembly for use in defining acavity within a molded article, comprising: at least one pair of spacedapart mold members each having a first surface which defines a portionof the cavity; an actuating member received within the space betweensaid spaced apart mold members and movably joined to said mold memberswith means allowing said actuating member a predetermined extent ofmovement with respect to said mold members; first cam means connectedwith each of said mold members; second cam means disposed on saidactuating member in complementary operative relation with said first cammeans, said first and second cam means being operative to extend saidmold members to a position of maximum spacing responsiVe to movement ofsaid actuating member to a first position in said predetermined extentof movement and being operative to retract said mold members to aposition of lesser spacing responsive to movement of said actuatingmember to a second position in said predetermined extent of movement;first abutment means disposed on each of said mold members in facingrelation with said actuating member; second abutment means disposed onsaid actuating member in facing relation with said spaced apart moldmembers, said second abutment means positioned on said actuating memberto be offset from said first abutment means when said actuating memberis in said second position and to be in abutting relation with saidfirst abutment means to relieve force exerted against said mold membersfrom transmission to said actuating member through said cam means whensaid actuating member is in said first position.
 6. An expandable coreassembly as in claim 5, wherein said first abutment means comprises atleast one wedge surface rigidly affixed to each of said pair of moldmembers; and said second abutment means comprises corresponding wedgesurfaces rigidly affixed to said actuating member in position to moveinto wedging abutment with said at least one wedge surface of each ofthe mold members when said actuating member is in said first position.7. An expandable core assembly as in claim 6, wherein said firstsurfaces of said mold members are proximately parallel with respect toeach other.
 8. An expandable core assembly for use in an injectionmolding apparatus for defining a cavity within a molded article,comprising: a first pair of spaced apart mold members each having asurface which defines a portion of the cavity; a second pair of spacedapart mold members each having a surface which defines another portionof the cavity; an actuating member received within the space betweensaid first and second pairs of mold members and moveably joined to eachof said mold members with means allowing said actuating member apredetermined extent of movement with respect to said mold members, saidfirst and second pairs of mold members being retained by said actuatingmember in predetermined spaced relation so that the space between saidpairs defines a region within which said actuating member is received;first cam means connected with each of said first pair of mold members;second cam means disposed on said actuating member in complementaryoperative relation with said first cam means; third cam means connectedwith each of said second pair of mold members; fourth cam means disposedon said actuating member in complementary operative relation with saidthird cam means; said first and second cam means being operative toextend said first pair mold members to a position of maximum spacingresponsive to movement of said actuating member to a first position insaid predetermined extent of movement and said third and fourth cammeans being operative to extend said second pair of mold members to aposition of maximum spacing responsive to movement of said actuatingmember to said first position; said first and second cam means beingoperative to retract said first pair of mold members to a position oflesser spacing responsive to movement of said actuating member to asecond position in said predetermined extent of movement, and said thirdand fourth cam means being operative to retract said second pair of moldmembers to a position of lesser spacing responsive to movement of saidactuating member to said second position; first abutment means disposedon each of said first pair of mold members in facing relation with saidactuating member; second abutment means disposed on said actuatingmember in facing relation with said spaced apart first pair of moldmembers; third abutment means disposed on each of said second pair ofmold members in facing relation with said actuating member; fourthabutment means disposed on said actuatinG member in facing relation withsaid spaced apart second pair of mold members; said second abutmentmeans positioned on said actuating member in offset relation with saidfirst abutment means when said actuating member is in said secondposition and in abutting relation with said first abutment means whensaid actuating member is in said first position; and said fourthabutment means positioned on said actuating member in offset relationwith said third abutment means when said actuating member is in saidsecond position and in abutting relation with said third abutment meanswhen said actuating member is in said first position; each of saidabutment means in said abutting relation being cooperative to relieveforce exerted against said mold members from transmission to saidactuating member through said cam members.
 9. An expandable coreassembly as in claim 8, wherein said surfaces of said first pair of moldmembers are parallel with respect to each other, and said surfaces ofsaid second pair of mold members are parallel with respect to eachother, and are positioned at a substantial angle relative to saidsurfaces of said first pair of mold members.
 10. An expandable coreassembly as in claim 8, wherein: said first and second abutment meanscomprise at least one set of complementary wedge surfaces rigidlyaffixed to each member of said first pair of mold members and to saidactuating member, respectively; said third and fourth abutment meanscomprise at least one set of complementary wedge surfaces rigidlyaffixed to each member of said second pair of mold members and to saidactuating member, respectively; said complementary wedge surfaces onsaid actuating member being positioned to move into wedging abutmentwith said respective complementary wedge surfaces on said mold memberswhen said actuating member is in said first position.
 11. An expandablecore assembly as in claim 8, wherein: said surfaces of said first pairof mold members have edges positioned in adjacent relation with edges ofsaid surfaces of said second pair of mold members to define a corevolume which is substantially enclosed around said pairs of surfaces;the surfaces of one of said pairs of mold members including fin edgemeans which closely overlap the adjacent edges of the surfaces of theother pair of mold members when said pairs of mold members are in theextended position; said fin edge means being resiliently yieldable inresponse to injection molding pressure to move into sealing engagementwith said adjacent edges of said other pair of mold members.
 12. Anexpandable core assembly as in claim 8, further comprising fluid flowpassageway means disposed within said mold members to guide a flow ofcoolant fluid in heat exchange relation with said mold members.